Method and means for the production of columnar-grained castings

ABSTRACT

A method and materials for producing shell molds for casting columnar grained metal components wherein a pattern is prepared corresponding to the shape of casting to be formed. The pattern is then combined with a base formed of a material selected from the group consisting of graphite and a refractory of high thermal conductivity. A shell mold is formed around the assembly comprising the pattern and base followed by removal of the pattern material to provide the shell mold. The base operates to conduct heat away from molten metal poured therein at a rate greater than the surrounding mold walls whereby nucleation will begin in the area adjacent the base and columnar grain growth will occur. The base included in the assembly of the pattern and base can also be used for connecting supporting rods to lend stability to an arrangement which includes a plurality of the assemblies whereby dip coating for formation of the shell mold can proceed without the need for a supporting baseplate.

D United States Patent 1 1 3,598,167

[72] Inventor Michael H. Snyderman 3,441,078 4/1969 Chandley 164/361 XPardppany, NJ. 3,131,999 5/1964 Suzuki et al. 164/36 Q E 1 PrimaryExaminer-J. Spencer Overholser Pacmed b Assistant Examiner-V, K. Rising1 [73] Assignee United Aircraft Corporation Auomey chafles wane EastHartford, Conn.

[54] METHOD AND MEANS FOR THE PRODUCTION OF COLUMNAIt-GRAINED CASTINGS11 Claims, 4 Drawing Figs.

[52] [LS-Cl 164/26, 164/24, 164/34, 164/35, 164/127, 164/236,164/246,164/361, 164/353, 164/371 [51 1 Int. Cl B22c 9/00 ABSTRACT: Amethod and materials for producing shell molds for casting columnargrained metal components wherein a pattern is prepared corresponding tothe shape of casting to be formed. The pattern is then combined with abase formed of a material selected from the group consisting of graphiteand a refractory of high thermal conductivity. A shell mold is formedaround the assembly comprising the pattern and base followed by removalof the pattern material to provide the shell mold. The base operates toconduct heat away from molten metal poured therein at a rate greaterthan the surrounding mold walls whereby nucleation will begin in thearea adjacent the base and columnar grain growth will occur.

The base included in the assembly of the pattern and base can also beused for connecting supporting rods to lend stability to an arrangementwhich includes a plurality of the assemblies whereby dip coating forfonnation of the shell mold can proceed without the need for asupporting baseplate.

METHOD AND MEANS FOR THE PRODUCTION OF COLUMNAR-GRAINED CASTllNGS Thisinvention relates to an improved system for the production of componentscharacterized by a columnargrained structure. The invention will bespecifically described relative to the production of turbine blades andvanes employed in gas turbine power plants. It will be understood,however, that the invention is also applicable to the production ofcomponents which might be subjected to similar operating conditions orwhich are more satisfactorily characterized by a columnar-grainedgrowth.

The provision of turbine components having a columnargrained growth haspreviously been described, for example in Ver Snyder Pat. No. 3,260,505,issued July 12, 1966. As explained in this patent, turbine componentshaving columnargrained growth exhibit a longer life before rupture andhave greater uniformity of stress rupture life when compared under thesame conditions with components of the same alloy but having anonoriented grain growth. The previously described procedure alsoprovides blades having third stage creep so that predictable changes increep" rate can be utilized in estimating time between engine overhauls.The components also have especially good tensile elongation properties,and can be employed at substantially higher temperatures than prior, conventionally cast blades. Completely satisfactory ductility, elongation,and extended life properties are obtained along with unusually uniformstrength properties and relatively high tensile strength.

In accordance with the previously described procedure, the alloy to becast is vacuum melted and poured into a mold. The mold is preferably aceramic or siliceous material such as a shell mold which is formed ofalternate layers of finely divided siliceous material, such assilicates, zirconia, or other argillaceous or refractory material, andfinely divided sand or like material. Several layers of the ceramic orsiliceous material and of the sand are usually employed. The shell moldsare usually formed on a wax pattern with the mold being fired duringwhich time the wax is removed.

In order to achieve the columnar-grained growth, Ver Snyder teaches theprovision of heating means for maintaining the upper portion of the moldat a temperature of about 100 F. above the melting point of the alloybeing cast. The mold is characterized by an open bottom which is locatedon a support member maintained substantially colder than the body of themold during casting. The chilling of the cast metal at one end causesthe component being cast to crystallize in a columnar structure whereinthe crystals are unidirectionally oriented and are aligned substantiallyparallel to the vertical axis of the component. The presence of grainboundaries normal to the stress axis of the cast component issubstantially avoided.

Other methods have also been described for achieving differentialcooling rates in molds to thereby promote columnargrained growth. InChandley Pat. No. 3,248,764, reference is made to the use of metalshells which are located around the mold at different levels to providea thermal gradient. Chandley Pat. No. 3,376,915 refers to the use ofinduction heating means which can be partially and selectivelydeenergized for providing a thermal gradient.

It is a general object of the instant invention to provide an improvedsystem for achieving columnar-grained growth in cast components.

It is a more specific object of this invention to provide a castingmethod and apparatus which will enable the production ofcolumnar-grained growth in cast components by highly efficient means inthat the cast products can be produced at relatively low cost with asignificant reduction in rejects due to the presence of inclusions inthe cast products.

These and other objects of this invention will appear hereinafter andfor purposes of illustration, but not of limitation, specificembodiments of the invention are shown in the accompanying drawings inwhich:

FIG. 1 is an elevational view, partly in section, illustrating themanner in which an assembly of a wax pattern and conductive base isformed for use in the practice of the invention;

FIG. 2 is a vertical sectional view of an assembly of the type shown inFIG. 1 having a shell mold formed therearound;

FIG. 3 is an elevational view of a prior art arrangement for preparingpatterns for the application of a shell mold; and,

FIG. 4 is an elevational view of an improved arrangement for theformation of a shell mold around wax patterns.

The instant invention includes a method which involves the preparationof a pattern conforming in shape to the columnargrained metal componentto be cast. The pattern material is associated with a base formed of amaterial characterized by high heat conductivity and by the ability towithstand the casting temperatures. A material consisting of graphite orof high thermal conductivity refractory will serve as a suitablematerial for forming the base.

A mold is invested around the assembly comprising the pattern and baseand thereafter the pattern material is removed from the mold.Conventional pattern and mold materials may be employed, for example,wax may be utilized for forming the pattern and shell molding techniquesemployed for building up a mold around the assembly of the pattern andbase. After removal of the wax, molten metal can be introduced into themold with the base and shell mold walls defining the configuration ofthe component being cast. Because of the high thermal conductivity ofthe base, heat will be removed at a relatively high rate in this areathereby promoting nucleation of the metal adjacent the base surface andthereby promoting a columnar-grained growth in a direction extendingaway from the base.

The novel means of the invention comprises the assembly of the patternmaterial and base. In a preferred form of the invention, the base andpattern assembly are produced by forming (as by molding) one or theother of these elements in direct association with the remainingelement. With this arrangement, the base will define the exactconfiguration of the end of the pattern and, accordingly, the castcomponent will have a substantially finished configuration therebyreducing machining and other finishing costs. The system of thisinvention also simplifies the provision of nucleating means inassociation with the base, and the use of the graphite or ceramic basesubstantially reduces the possibility of inclusions being formed in thecasting.

The invention also contemplates the provision of a novel arrangement foruse during investing of mold material in a shell molding process.Specifically, the base portions of a plurality of assemblies comprisingthe pattern and base can be interconnected through the use of rod means.This arrangement will also include pattern material for forming gatesand sprues whereby the entire arrangement can be provided with asurrounding shell mold through alternate applications of dip coat andrefractory material. The use of the supporting rods in conjunction withthe base portions of the individual assemblies permits elimination of asupporting base plate thereby simplifyingthe shell molding procedure.

FIG. 1 illustrates a wax pattern 10 which conforms in shape to theturbine element in the cast. The wax pattern includes a base portion 12defining recessed areas 14.

A mold wall 16 extends around the end portion 12 of the wax pattern.This mold wall thereby provides a mold cavity in conjunction with theend portion 12, and a base 18 is adapted to be formed in this cavity. Aslurry of material may be employed for filling the mold cavity or otherstandard techniques for forming components from graphite or ceramicmaterials may be utilized. At any rate, the preferred form of theinvention calls for the combination with the pattern of a base 18 whichconforms with the end of the pattern and which thus conforms with theend of the component to be ultimately cast.

As an alternative to the arrangement shown in FIG. l, the base 18 couldbe formed in a mold specifically designed for forming the configurationillustrated. This base 18 could then be associated with a patternforming mold whereby the pattern including the end portion 12 wouldbecome associated with the base 18. As a further alternative, the basecould be machined to the desired configuration and then joined with thewax pattern.

The assembly comprising the base 18 and the pattern 10 may then beprovided with a surrounding shell mold 20 as shown in FIG. 2. Standardtechniques may be employed for forming the shell mold. Ceramic pin means22 may be utilized for insuring a good connection between the base 18and the shell mold 20. This arrangement can be accomplished by drillingholes in the base 18 and then press fitting the pins 22 in these holes.The shell mold material will then be invested around the pins to providea secure connection. A layer of paraffin may be formed around theperiphery of the base 18 prior to investing to allow for expansion ofthe base during subsequent heating.

FIG. 3 illustrates one standard arrangement which may be employed forinvesting a shell mold around the assembly comprising the pattern 10 andbase 18. In this arrangement, an aluminum plate 24 is provided forsupporting the assemblies. A connecting rod 26 is inserted through thisplate and extends upwardly for connection with a handle 28. A top plate29, ceramic sprue cup 30, wax sprue 32, and wax gates 34 are connectedin conventional fashion. When supported by the rod 26 and the handle 28,this arrangement can be alternately dip coated and covered withrefractory for the formation of shell molding material around all of theexposed surfaces. The pattern material is then removed and the moldfired in preparation for the introduction of molten metal. Shell moldingtechniques of a type suitable for use are described, for example, inErdle and Feagin Pat. No. 3,005,244 issued Oct. 24, 1961.

Due to the relatively high thermal conductivity of the base 18,nucleation of the molten metal will occur on the base surface, andcolumnar-grained growth will proceed. The relatively high thermalconductivity of the base will provide solidification in this matter;however, cooling means may also be provided to insure solidification inthe desired fashion. Thus, the assemblies could be located on a metalsupport which includes internal cooling whereby heat will be morerapidly drawn through the base portion 18 for promoting the desiredsolidification. Nucleating, for example, oxides of cobalt or nickel,cobalt aluminate, or cobalt silicate, may be utilized to provideadditional assurances that nucleation will occur on the base surface.

FIG. 4 illustrates an alternative means for supporting the assembliescomprising a pattern 10 and base 18 prior to shell molding. In thisarrangement, a ceramic cup 38, wax sprues 40 and 42, and wax gates 44are employed in accordance with conventional practice. The wax gates 44extend to the pattern material 10. The base portions 18 associated withthe pattern material 10 are interconnected by means of a ceramic rod 46.The extension 48 of the sprue 42 extends to a central portion of thisrod primarily for purposes of supporting the rod during investing of amold around the arrangement of FlG. 4.

The rod 46 may be press-fitted in bores defined by the respective baseportions 18. The rod will, therefore, assist in holding the arrangementintact during investing and after removal of the pattern material fromthe invested mold, the rod will continue to provide support. Thus, theprovision of the base portions 18 provides a means for anchoring asupporting rod, and with this arrangement, the use of an aluminum setupplate, such as the support plate 24 shown in FIG. 3, can be eliminated.This simplified the setup prior to dip coating as well as the eventualcasting operation particularly when the base portions 18 are to beplaced in contact with a cold support during the casting operation.

It has been found that the formation of shell molds around the assemblycomprising pattern material 10 and base portion 18 will provide definiteadvantages over systems currently in use. The arrangement of thisinvention permits the use of patterns which have a configurationdirectly corresponding with the configuration of the component to beproduced. This provides a distinct saving in machining cost since theend portions of the cast components do not have to be machined to anyextensive degree.

The instant invention also simplifies the use of nucleating means sincethese means can be formed directly in conjunction with the formation ofthe base portions 18. Finally, the use of graphite or highly conductiverefractory materials for forming the base portion has been found tosubstantially reduce the presence of inclusions in the finished product.This substantially reduces the number of rejects and, therefore,increases the efficiency ofthe casting operation. Where the moltenmaterial is cast into direct contact with a shell block as inconventional practice, inclusions are a continuing problem and result ina relatively high rate of rejects.

The system of the invention greatly reduces the necessity for reworkingwax patterns since simpler wax assemblies can be used. The setupprocedures for an arrangement suitable for the formation of shell moldsare greatly simplified and the overall casting cycle time is shortenedbecause of the simplification and because hot molds can be employed withthe techniques ofthis invention.

As indicated, conventional materials and techniques can be utilizedduring formation of the patterns and shell molds. With respect to themold base, the material selected may be any highly heat-conductivematerials which are more highly conductive than the mold itself. Shapesmade from powdered metals, metallic carbides, borides, beryllides,graphite and heat-conductive glasses and ceramics would be suitable. Powdered metal shapes may include those formed from powdered tungsten,tantalum, copper, stainless steel and molybdenum with the specific metalshape being selected depending upon the metal being poured since themetal shape should be sufficiently stable to allow the particular alloyto be poured against it. Tungsten and tantalum carbides, titaniumdibordie, tantalum boride and beryllide, titanium nitride, boron nitrideand shaped beryllium oxide are examples of carbides, borides andberyllides which may also be employed.

It will be understood that various changes and modifications may be madein the above described system which provide the characteristics of thisinvention without departing from the spirit thereof particularly asdefined in the following claims.

That which I claimed is:

1. A method for the production of a cast columnar-grained metalcomponent comprising the steps of preparing a pattern corresponding tothe shape of the casting to be formed and having a bottom surface,forming against the bottom surface of said pattern a base formed of amaterial selected from the group consisting of graphite and high thermalconductivity refractory, said base having a surface corresponding inshape to the base surface of the pattern and in contact therewith,building up mold walls of sufficient thickness and strength about theassembly comprising the pattern and the base to form a shell moldthereabout, leaving exposed the surface of the base opposite to thepattern and then removing the pattern material from the mold to providea shell mold, said base forming the bottom wall of the mold andoperating to remove heat from molten metal poured into the formed shellmold at a rate greater than the surrounding mold walls to promotenucleation of the metal adjacent the surface of the base.

2. A method in accordance with claim I wherein said base is formed to aconfiguration conforming with the bottom wall of the pattern and thesurface of the base opposite to the bottom wall of the pattern isexposed by the mold for effective heat removal through said base.

3. A method in accordance with claim 2 wherein said assembly is formedby first preparing said pattern, providing mold walls around the bottomof the pattern whereby the pattern bottom serves as the bottom wall ofsaid last mentioned mold, and introducing material for forming said baseinto said last mentioned mold for securing the base to the pattern.

4. A method in accordance with claim 2 including the step of providing amold conforming to the shape of said base, molding said base, andthereafter applying said pattern material to said base for forming theassembly comprising the pattern and base.

5. A method in accordance with claim I wherein a shell molding processis followed for investing said mold around said pattern and base 6. Amethod in accordance with claim 1 including the step of locatingnucleating agents on said base prior to the introduction of molten metalinto said mold.

7. A construction for the production of cast columnargrained metalcomponents comprising an assembly consisting of a disposable patterncorresponding to the shape of the casting to be formed and having abottom wall and a base engaging and conforming to the bottom wall ofsaid pattern, said base being selected from the group consisting ofgraphite and high thermal conductivity refractory, and a shell moldbuilt up around said assembly, leaving exposed as the bottom wall of themold the surface of the base opposite to the pattern.

8. A construction in accordance with claim 7 wherein said base is formedby solidification against the bottom wall of the pattern to aconfiguration conforming with the bottom wall of the pattern.

9. A construction in accordance with claim 7 wherein a plurality of saidassemblies are secured together prior to application of the moldmaterial, and including supporting rod means extending between the baseportion of the assemblies, said rod means being formed of nondisposablematerial whereby the rod means operate to support the respective baseportions relative to the mold after removal of the pattern material fromthe mold.

10. A construction in accordance with claim 7 including pin meanssecured to said base and extending outwardly therefrom, said pin meansextending into the mold material invested around the base for therebysecuring said mold relative to said base.

11. A method in accordance with claim 1 including the step ofpositioning the mold on a chill plate with the base in contact with thechill plate prior to introducing molten metal into the mold. t

1. A method for the production of a cast columnar-grained metalcomponent comprising the steps of preparing a pattern corresponding tothe shape of the casting to be formed and having a bottom surface,forming against the bottom surface of said pattern a base formed of amaterial selected from the group consisting of graphite and high thermalconductivity refractory, said base having a surface corresponding inshape to the base surface of the pattern and in contact therewith,building up mold walls of sufficient thickness and strength about theassembly comprising the pattern and the base to form a shell moldthereabout, leaving exposed the surface of the base opposite to thepattern and then removing the pattern material from the mold to providea shell mold, said base forming the bottom wall of the mold andoperating to remove heat from molten metal poured into the formed shellmold at a rate greater than the surrounding mold walls to promotenucleation of the metal adjacent the surface of the base.
 2. A method inaccordance with claim 1 wherein said base is formed to a configurationconforming with the bottom wall of the pattern and the surface of thebase opposite to the bottom wall of the pattern is exposed by the moldfor effective heat removal through said base.
 3. A method in accordancewith claim 2 wherein said assembly is formed by first preparing saidpattern, providing mold walls around the bottom of the pattern wherebythe pattern bottom serves as the bottom wall of said last mentionedmold, and introducing material for forming said base into said lastmentioned mold for securing the base to the pattern.
 4. A method inaccordance with claim 2 including the step of providing a moldconforming to the shape of said base, molding said base, and thereafterapplying said pattern material to said base for forming the assemblycomprising the pattern and base.
 5. A method in accordance with claim 1wherein a shell molding process is followed for investing said moldaround said pattern and base.
 6. A method in accordance with claim 1including the step of locating nucleating agents on said base prior tothe introduction of molten metal into said mold.
 7. A construction forthe production of cast columnar-grained metal components comprising anassembly consisting of a disposable pattern corresponding to the shapeof the casting to be formed and having a bottom wall and a base engagingand conforming to the bottom wall of said pattern, said base beingselected from the group consisting of graphite and high thermalconductivity refractory, and a shell mold built up around said assembly,leaving exposed as the bottom wall of the mold, the surface of the baseopposite to the pattern.
 8. A construction in accordance with claim 7wherein said base is formed by solidification against the bottom wall ofthe pattern to a configuration conforming with the bottom wall of thepattern.
 9. A construction in accordance with claim 7 wherein aplurality of said assemblies are secured together prior to applicationof the mold material, and including supporting rod means extendingbetween the base portion of the assemblies, said rod means being formedof nondisposable material whereby the rod means operate to support therespective base portions relative to the mold after removal of thepattern material from the mold.
 10. A construction in accordance withclaim 7 including pin means secured to said base and extending outwardlytherefrom, said pin means extending into the mold material investedaround the base for thereby securing said mold relative to said base.11. A method in accordance with claim 1 including the step ofpositioning the mold on a chill plate with the base in contact with thechill plate prior to introducing molten metal into the mold.